The present invention relates to an improved membrane and a process for producing improved membranes.
It is well known to use zeolites and similar crystalline zeolitic materials in separations and as catalysts. Zeolitic membranes and membranes which incorporate zeolites are also well known and can come in a range of different types. European Patent Application 0481660 discloses and discusses prior art zeo-type membranes and refers in particular to U.S. Pat. Nos. 3,244,643, 3,730,910 and 4,578,372, Applied Catalysts 49(1989) 1-25, DE-A-3827049, CA1235684, JP-A-63287504, JP-A-63291809, EP-A-180200, EP-A-135069.
It is disclosed in EP 0481 658 A1 to surface coat a porous support on which a zeo-type material is deposited by crystallisation from a synthesis gel with a surface coating of nickel cobalt or molybdenum in the form of the metal and/or oxide.
The methods disclosed for forming this surface coating include vapour deposition, vacuum evaporation, Rf sputtering or electroplating or deposition of a salt from a liquid and oxidising the salt. These methods give a surface coating on the porous support of varying thicknesses and provide for improved crystal deposition from the gelmainly round the wires of the support with little improvement in filling the voids after a single growth. EP 0481 659 A1 discloses a similar process for pre-treating a porous support, except that the porous support is pre-treated with an acid.
The process of EP 0481660 A1 discloses treating the porous support a plurality of times with the synthesis gel and crystallising a zeo-type material from tinc gel in order to obtain an improved coating. However, this process can leave gel and other debris blocking the pores of the porous support and thus prevent complete coverage, even wiping between zeolite growths will leave debris behind.
However, these processes do not produce a defect free membrane without a plurality of growths and they do not bridge the voids after a single growth, and even though these patent applications disclose repeated retreatment with the gel as being required to block xe2x80x9cpin holesxe2x80x9d, these existing methods have not proved successful. Even small defects or pinholes can have a marked deleterious effect on the performance of membranes and can render them of substantially little value in many operations. This is because in many separation operations the effect of defects is essentially to provide a channel where the unseparated products can pass.
Some existing methods claim that a defect tree membrane is obtained on a laboratory scale, but attempts to provide a substantially defect free membrane on a larger scale have proved unsuccessful.
In order to provide an imp roved membrane with better performance characteristics, we have devised a treatment for such membranes.
According to the invention, there is provided a process for treating a membrane comprising a film of a crystalline zeo-type material which process comprises treating the membrane with a silicon or other metal compound capable of reacting with the membrane either before, after or instead of the treatment of the membrane with a silicic acid or polysilicic acid as set out in patent application PCT/GB95/00956.
Zeo-type materials are also known as molecular sieves which are widely known and used. They comprise an extended network of channels formed from silicon/oxygen tetrahedrons joined through the oxygen atoms. Zeolites and alumino-silicates are the most commonly known form of zeo-type materials and the present invention is applicable to any membrane formed from zeo-type materials and particularly applicable to zeolites and alumino-silicates. In the xe2x80x9cAtlas of Zeolite Structure Typesxe2x80x9d, Meier and Ofsen, 1987, Polycrystal Book Service, Pittsburg USA, various types of structure are described and, for example, those described as having LTA, MEL, MFI or TON structure can be used.
In xe2x80x9cNew Developments in Zeolite Science and Technology Proceedings of the 7th International Conference, Tokyo, 1986, page 103, another class of zeo-type materialsare disclosed as crystalline aluminophosphate, silicoalumina phosphates and other metallo-alumino phosphates.
Typical zeolites which can be used in the present invention are include but are not limited to, 3A, 4A, 5A, 13X, X, Y, ZSM5, MPOs, SAPOs, Silicalite, xcex2 or theta or theta-1, etc.
The porous supports on which zeo-type membranes are formed and which can be used in the present invention include those formed of metals, ceramics, glass, mineral, carbon or polymer fibres or cellulosic or organic or inorganic polymers. Suitable metals include titanium, chromium and alloys such as those sold under the Trade Marks xe2x80x9cFecralloyxe2x80x9d and xe2x80x9cHastalloyxe2x80x9d and stainless steels. The porous supports may be formed of a mesh or from sintered metal particles or a mixture of both. These are commonly sold in the form of filters.
Porous ceramics, glass mineral or carbon materials can be used including porous silicon and other carbides, clays and other silicates and porous silica. If desired, the support can be a zeolite formed by compression or using a binder, or by the conversion of meta kaolin to a zeolite. The shape of the support is not critical, for example, flat sheet, tubular, wound spiral, etc. can be used. If polymeric materials are used, these can optionally be film coated with metal or metal oxide or a silicic acid as herein defined.
The porous support can be also be a granular solid e.g. formed of particles of a closely packed material such as a pellitised catalyst.
The present invention can be used with porous supports of any suitable size although, for large flux rates through a membrane, large pore sizes are preferred. Preferably pore sizes of 0.01 to 2,000 microns, more preferably of 0.01 to 200 and ideally of 0.01 to 5 microns are used. Pore sizes up to 300 microns can be determined by bubble point pressure as specified in ISO 4003. Larger pore sizes can be measured by microscopic methods. The larger the relative amount of the surface which is composed of voids in general the more suitable the porous support.
The membranes which can be treated by the method of the present invention can be formed by any method, for example by crystallisation from a gel or solution, by plasma deposition or by any other method such as electrodeposition of crystals on conducting substrates e.g. as described in DE 4109037 or the conversion of meta kaolin to a zeolite.
When the membrane comprising a film of zeo-type material is prepared by crystallisation from a synthesis gel, any of the methods described in the prior art can be used.
The synthesis gel used in the process can be any gel which is capable of producing the desired crystalline zeo-type material. Gels for the synthesis of zeo-type materials are well known and are described in the prior art given above or, for example, in EP-A-57049, EP-A-104800, EP-A-2899 and EP-A-2900. Standard text books by D W Breck (xe2x80x9cZeolites Molecular Sieves, Structure Chemistry and Usexe2x80x9d) published by John Wiley (1974) and P. A Jacobs and J. A Martens (Studies in Surface Science and Catalysis No. 33, Synthesis of High Silica Alumino silicate Zeolitesxe2x80x9d published by Elsevier (1987), describe many such synthesis gels. The process which can be used includes conventional syntheses of zeo-type materials, except that the synthesis is carried out in the presence of the porous support. Most commonly, gels are crystallised by the application of heat.
The membrane which is treated by the process of the invention can be prepared by a process which comprises deposition or crystallisation from a Larger pore medium. In one embodiment of the invention the growth medium can be used in two different methods. In the gel method (method 1) for forming the membrane the gel used to form the membrane preferably has a molar composition in the range of
(1.5-3.0)Na2O:(1)Al2O3:(2.0)SiO2:(50-200)H2O
and the method used can be used in any of the methods disclosed in the references listed above. In the liquid solution method (method 2) the liquid solution used to form the membrane preferably has a molar composition in the range of:
(6-10.0)Na2O:(0.2)Al2O3:(1.0)SiO2:(150-250)H2O
The liquid solution preferably contains a maximum amount of the compound capable of crystallising to form a zeo-type material whilst still remaining a liquid solution. By maximum amount is meant the maximum amount which can be maintained in solution so that no precipitation occurs before zeolite formation.
Methods (1) and (2) can be used under the conditions listed below and method (1) and method (2) can be used either on their own or with method (1) followed by method (2) or vice versa.
The conditions which can be used for forming the membrane are with a temperature of the growth solution preferably in the range of 50 to 100xc2x0 C. and the pH can be adjusted e.g. to pH of 12.5 to 14 by addition of sodium hydroxide or ammonia. If desired the sodium ion concentration can be increased without increasing the pH by the addition of a sodium salt such as sodium chloride. The growth solution can be seeded with zeolite crystals of the desired zeolite to be synthesised. The membrane can be washed to pH neutral after membrane formation prior to any post-treatment.
The porous support can be contacted with the growth medium by immersion or by pouring the growth medium over the support with the support held substantially horizontal, either face up at the bottom of a container, or face down at the surface of the growth medium, or it can be passed over one or both sides of the support, with the support held substantially horizontal, or it can be passed over one or both sides of the support with the support held substantially vertical or the support can be in any intermediate position or down the inside of a tubular support.
The growth medium can be kept static, stirred, tumbled or passed over or around the support, alternatively the growth medium can be passed over both sides of the support or vertically up and down the inside or outside of a tubular support with the support held substantially horizontal or at any intermediate position.
Pressure may also be applied but it is usually convenient to conduct the crystallisation under autogenous pressure. Preferably the porous support is completely immersed in the growth medium; alternatively, if desired, only one surface of the support may be in contact with the growth medium.
This may be useful, for example, if it is desired to produce a membrane in the form of a tube, where only the inside or outside of the tube need be in contact with the growth medium or the growth medium can be contacted with the inside or outside of a vertical tubular support.
It may be useful if it is desired to produce a membrane containing two different zeolites, one on each side of the support. Use of such a bi-functional membrane would be equivalent to using two separate membranes, each carrying a different zeolite.
If desired, the treatment with the gel or liquid solution can be repeated one or more times to obtain thicker membrane coatings and/or more coherent coatings.
Preferably the porous support is pre-treated with a zeolite initiating agent. The zeolite initiating agent is preferably a cobalt, molybdenum or nickel oxide or it can be particles of a zeolite, e.g. the zeolite which it is intended to deposit on the porous support, or any combination of these. Another example of an initiating agent is a compound which can deposit a zeo-type pre-cursor material e.g. a silicic acid or polsilicic acid.
The zeolite initiation agent can be contacted with the porous support by a wet or dry process. If a dry process is used, the particles of the zeolite initiation agent can be rubbed into the surface of the porous material, or the porous material surface can be rubbed in the particles.
Alternatively the particles of the zeolite initiation agent can be caused to flow over and/or through the porous support, or pulled into the support by means of a vacuum.
If a wet process is used, a liquid suspension of powder of the zeolite initiation agent is formed and the liquid suspension contacted with the porous support to deposit the zeolite initiation agent on the support.
Before contacting the surface of the porous support with the zeolite initiation agent the surface is preferably wetted with wetting agent such as an alcohol, water or a mixture of these.
When a silicic acid is used as an initiating agent it can be a silicic acid as herein defined.
In the present specification by silicic acid is meant monosilicic, low, medium and high molecular weight polysilicic acids and mixtures thereof.
Preferred compounds used to treat the membrane according to the present invention include silicon, titanium, zirconium or any metal compound which can form a solution or gel and which can condense to form a polymeric type structure. For instance silicon containing compounds can include silanes, silicates and organic silicone polymers.
The preferred silanes are chloro, alkoxy, or any silane with a reactive group functionality which can undergo a condensation reaction.
The preferred silicates are those which have easily removable cations, e.g. mon- and di-valent cations.
The treatment with the silicon containing compound depends on the state of the silicon containing compound. If it is gaseous then the membrane can be contacted with the silicon containing compound in the gaseous state, preferably at an elevated temperature e.g. of above 0xc2x0 C. and more preferably above 20xc2x0 C.
If the silicon containing compound is a solid then it can be dissolved in a suitable solvent such as tetrahydofuran, ethanol, butanol or any protonated solvent and the membrane contacted with the solution.
If the silicon containing compound is a liquid then the membrane can be contacted with the liquid or a mixture of the liquid with a suitable solvent.
The other metal compounds other than silicon compounds useful in the present invention can be used in the form of a solution or gel of a metal oxide, alkoxide or hydroxide. Suitable metals include titanium, zirconium or any metal which can form a solution or gel and which can condense to form a polymeric type structure e.g. a suitable oxide, hydroxide, or alkoxide.
The membranes treated by this process are improved in terms of their performance and membrane strength compared with untreated membranes.
The membranes formed using the present invention can be used in a range of separation and catalytic processes, e.g. dehydration of LPG, air, alcohols and natural gas, removing linear alkanes, olefins and substituted hydrocarbons from mixtures with branched chain compounds, e.g. in reforming, dewaxing, etc., hydrogenation and dehydrogenation of linear hydrocarbon in admixture with branched chain compounds.